X-ray fluorescence (XRF) instruments measure properties of material by irradiating the material with x-rays or gamma rays and analyzing the fluorescent radiation to determine specified properties. The term “x-rays”, as used herein and in any appended claims, refers to radiation that is generated either by radioactive sources, or by instruments such as x-ray tubes, and encompasses within the term all forms of penetrating radiation including gamma rays. The specified properties to be determined may include the elemental composition of the irradiated object, or the distribution of a particular element in the near surface of the object, or the density of the object, or the morphology.
XRF instruments typically have collimated beams and appropriate shielding so that the operator is not subjected to undue ionizing radiation. For example, laboratory XRF instruments typically require the operator to completely cover the instrument and the sample so that negligible radiation emanates from the XRF instrument.
Portable XRF instruments have special radiation shielding requirements since their use typically requires that the operator hold the instrument while making the measurements. The ambient radiation levels are a primary concern. The operator and any nearby people must not be subject to undue levels of ionizing radiation. XRF instruments that inspect houses for lead paint are the specific embodiment of this invention and offer a good example of its need.
Portable XRF instruments are now the choice for quantitative determinations of the concentration of lead in painted walls of a house. Commercial portable XRF lead-paint instruments use either radioactive sources, such as 109Cd and 57Co, or x-ray tubes to generate the fluorescing radiation that excite the lead atoms in the painted surfaces. The intensity of the fluoresced characteristic x-rays of lead gives measure to its concentration and allows the inspector to determine whether the paint is out of compliance with established regulatory limits.
The allowable ambient radiation levels differ from country to country. The United States regulations place restrictions on the radiation levels in the ambient space directly behind the instruments x-ray exit port. Of special concern is the space where the operator may have his hands or face. Minimal attention is paid to the radiation levels in the space between the wall being inspected and the surfaces of the operator's hands, arms and body when taking the measurements. The radiation limitations in the US can be satisfied by applying shielding in the instrument itself.
Radiation limitations in Europe are currently significantly more stringent than those in the United States. The acceptable level of radiation for an occupation worker is ten times lower; that is, 1 μSv/hr for Europe and 10 μSv/hr for the US. (μSv/hr is the standard abbreviation for microSievert per hour, a level of radiation equivalent to 100 microrem of radiation in now obsolete units.) Moreover, and of special importance to this invention, France requires that no point 10 cm from any accessible surface of the XRF instrument exceed the 1 μSv/hr level. That requirement cannot be satisfied with the shielding inside an XRF instrument.
Commercial hand-held x-ray fluorescing instruments have radiation absorbing material in the nose of the instrument. This absorbing material is designed to absorb radiation that comes directly from the source but is not going out through the exit port to strike the sample under study. This absorbing material also absorbs radiation that has been once-scattered so that the once-scattered radiation does not enter the detector and does not confound the measurement being made. The absorbing material in the nose of the inspection instrument, however, cannot prevent radiation that is multiply scattered such that it emerges from the target in a place and direction in such a way as to not intersect the nose of the instrument.